The present invention relates to a novel rubber-modified styrene-based copolymer or, more particularly, to a rubber-modified styrene-based copolymer suitable as a molding material in injection molding, extrusion molding, extrusion-vacuum forming and the like and capable of giving a molded article having good appearance or, in particular, good appearance in the welded portion as well as excellent rigidity and impact strength.
It is widely practiced in the prior art in the industry of synthetic resins with an object to impart a styrene-based resin with improved impact strength that a rubber-modified polystyrene resin composition is prepared either by blending a polystyrene with a rubbery polymer or by polymerizing styrene in the presence of a rubbery polymer. In the latter method of polymerization, the molecules of the rubbery polymer are grafted with a part of the styrene while the remaining part of the styrene is polymerized into polystyrene so that the resultant product is a composite of the polystyrene and the graft copolymer of styrene on the rubbery polymer.
On the other hand, known rubber-modified styrene-based copolymers include a so-called ABS resin which is a rubber-modifind copolymer of styrene and acrylonitrile. ABS resins have advantages that a very strong adhesion is obtained between the surface of the resin article and a chemical plating layer formed thereon in addition to the greatly improved impact strength and mechanical strengths as compared with polystyrenes as well as excellent moldability so that they are employed in a wide field of applications including machine parts, parts of electric and electronic appliances, parts of automobiles, household commodities, stationery goods and the like.
Rubber-modified copolymers of styrene and acrylonitrile are prepared usually by the method such as bulk-suspension polymerization method, continuous bulk polymerization method, emulsion polymerization method and the like. The methods of bulk-suspension polymerization and continuous bulk polymerization are advantageous, when comparison is made with the emulsion polymerization method performed by using a large amount of emulsifiers, in respect of the excellent productivity to give a high economic merit in addition to the advantage that the post-treatment subsequent to the polymerization involves little problem in the water pollution. A problem in the bulk-suspension polymerization method and continuous bulk polymerization method, however, is that the ABS resin obtained thereby is poor in respect of the balance of properties between appearance and the rigidity or mechanical strengths of the shaped articles.
Proposals have been made to provide an improved rubber-modified styrene-based copolymer free from the above mentioned problems by using a polybutadiene of low solution viscosity (see Japanese Patent Kokai 63-199717) and by using a blockwise copolymeric rubber of S-B type having a low solution viscosity (see Japanese Patent Kokai 63-207863). The former method by using a polybutadiene of low solution viscosity is not always quite satisfactory because the rubber-modified styrene-based copolymer prepared thereby gives shaped articles sometimes having a low rigidity and poor appearance, in particular, in the welded portions. As to the appearance of shaped articles in the welded portions, satisfactory rubber-modified styrene-based copolymers can not always be obtained by the latter method by using an S-B type blockwise copolymeric rubber having a low solution viscosity.